The subject matter of the invention is construction parts for gaseous hydrogen isotope containing media, especially pipes for the transfer of process heat which are made of nickel-based alloys containing 40 to 70% nickel, 15 to 30% chromium, 0 to 20% cobalt, 5to 10% molybdenum, and 0 to 20% iron.
For example, in the enrichment of coal, in methane reformation with steam or in petrochemistry, there are required construction materials which in addition to a good high temperature resistance at 850.degree. to 1100.degree. C. and oxidation resistance also must have a resistance to permeating hydrogen atoms. There are reasons to take the necessary energy for the enrichment of coal from nuclear sources, for example, from a high temperature reactor employing helium as heat carrier. Since this heat carrier also can contain the radioactive hydrogen isotope tritium, whose passage into the product gas of the enrichment of coal must be prevented as far as possible, there are required for the usable construction materials to an especial degree high resistance to permeation at high temperatures.
The permeation values for hydrogen or tritium with known, for the most part nickel and chromium containing, high-temperature materials are very neat to each other, but these values are impermissibly high, above all when employed in the nuclear area.
Therefore, it has already been proposed to provide these types of high temperature materials with a hydrogen permeation-retarding oxide layer.
In German OS 3104112, there is described an oxide coating which is produced in general on all high temperature alloys. However, there is the disadvantage that no definite resistance to the permeation of hydrogen can be produced, so that a practical employment of such material provided with an oxide coating is problematical because of the strongly fluctuating retardation of hydrogen permeation, likewise the adjustment of the thickness of the oxide coating to the permeation problem in each case in using the entire spectrum of high temperature alloys. In some cases, the resistance effect of oxide layers differs only insignificantly from the substrate. At the same substrate quality within the suggested frame of composition of high temperature alloys and at the same thickness of oxide coating furthermore there are in part considerable fluctuations in preventing permeation dependent on the tolerance permitted in the alloy composition and the impurities. The oxide coatings described in the above-mentioned patent publication and also in German OS 3108160 and 3215314, therefore, indeed are suited as corrosion protection coatings but are not reliable as hydrogen permeation barriers.
Therefore, the problem of the present invention is to provide construction parts for media containing gaseous hydrogen isotopes, especially pipes for the transfer of process heat made of nickel-based alloys containing 40 to 70% nickel, 15 to 30% chromium, 0 to 20% cobalt, 5 to 10% molybdenum, and 0 to 20% iron which lead to an optimum formation of fixed permeation-preventing barriers and, therefore, produce high quality of resistance to permeation for destined employment.